The invention relates to a display with a heater according to the precharacterizing clause of the first claim. According to this, the invention relates in particular to a display with a heater, the display having at least one display glass and the heater having an electrically operated heating element, the heating element comprising a thin, preferably transparent layer of a material forming an electrical resistance, the layer being applied over a surface area to the at least one display glass or being embedded over a surface area in the display.
Displays are being increasingly used, for example, in automotive engineering, for example LC displays (liquid crystal displays), in order to convey information to the driver. Such a display may be, for example, an integral part of an instrument cluster which is arranged in the dashboard of the vehicle. The display fitted in the vehicle is expected to be fully functional even at low ambient temperatures, although, for example in the case of an LC display, the contrast and, consequently, the quality of the display decrease markedly at low ambient temperatures. For this reason, displays fitted in a vehicle generally require a heater in order to heat them up to a suitable operating temperature within a short time when there are low ambient temperatures.
In the case of the heater in question here, the heating element heats up the display in accordance with the current intensity which occurs when the layer applied to the display and forming an electrical resistance is connected to a voltage supply. This voltage supply generally comprises the onboard electrical system of the vehicle, it being intended that the heating element can be connected to the onboard electrical system directly, i.e. without interconnecting a voltage-stabilizing circuit arrangement. With regard to the heat output which can be generated by the heating element, however, it must be taken into account that the level of the voltage present at the heating element may have different nominal values depending on the vehicle category and that the voltage of the onboard electrical system is subject to considerable fluctuations during the operation of the vehicle. Moreover, a heating element which comprises a thin layer applied to a display glass or a thin layer embedded in the display glass has a great range of variation in its resistance value, because both the layer thickness, and consequently the resistance per unit area, and the chemical composition of the material application are subject to relatively great tolerances in the production process. Because both the voltage present at the heating element and the value for the resistance forming the heating element vary considerably within certain limits, further measures are required for generating a heat output appropriate for requirements, which must take into account the currently prevailing ambient temperature to have an optimum effect. Otherwise, the heat generated may not be sufficient to make the display operational within a short time, or the display itself or its mounts, usually consisting of a plastic, may be destroyed by overheating.
It is consequently the object of the present invention to present a display with a heater, the heater ensuring a heat output which, in accordance with the ambient temperature and in spite of considerable fluctuations in the voltage supply to which the heating element is connected, and in spite of a range of variation in the resistance values of the heating element, makes the display operational in a short time even at low ambient temperatures.
The object is achieved by a display with a heater having the features of the first claim. In this case, the heater has an electrically operated heating element connected to a voltage supply, the display having at least one display glass and the heating element comprising a thin, preferably transparent layer, the layer consisting of a material forming an electrical resistance, the layer being applied over a surface area to the at least one display glass or being embedded over a surface area in the display. The solution achieving this is distinguished by the features
a) that a first measuring instrument is provided, which determines the resistance value of the heating element,
b) that a second measuring instrument is provided, which determines the level of the voltage present at the heating element,
c) that a third measuring instrument is provided, which senses the current ambient temperature at the display,
d) that a memory is provided, which stores the determined resistance value of the heating element,
e) that a logic unit is provided, which determines for the stored resistance value in accordance with the determined voltage and the determined ambient temperature a current-supplying period for the heating element necessary to produce that heat output which will heat up the display to a certain operating temperature within a predetermined time, and
f) that a switching element actuated by the logic unit is provided, which regulates the current flow through the heating element connected to the voltage supply for the current-supplying period determined.
As a result, the measures mentioned above provide the heater of a display operated in a vehicle with a control system which regulates the current flowing through the heating element in accordance with the measured ambient temperature, the actual resistance value of the heating element, the level of the voltage present at the heating element and also taking into account the required heat output to heat up the display to a certain operating temperature within a predetermined time. In this case, a logic unit influences the current flow through the heating element by means of a switching element.